A technique of modifying the resins with rubbers has conventionally been established to improve an impact strength of thermoplastic resins.
Examples of resins produced by the modification technique include ABS (Acrylonitrile-Butadiene-Styrene) resins obtained by reinforcing styrene-acrylonitrile copolymer resins (AS resins) with butadiene rubber and AAS resins obtained by reinforcing AS resins with an acrylic rubber. Although a silicone rubber may be thought to be usable as such a base rubber component, compositions obtained by merely blending thermoplastic resins with the silicone rubber have insufficient impact resistance because in such compositions which are a mere blend of a thermoplastic resin and a polyorganosiloxane, the two components show poor compatibility with each other. This problem may be overcome if the technique of grafting a vinyl monomer onto a rubber as ABS resins can be applied to modification with a polyorganosiloxane. However, polyorganosiloxanes generally have poor reactivity with vinyl monomers and it is difficult to obtain graft copolymers therefrom.
Various methods are disclosed for the production of a graft copolymer of the above kind. For example, JP-A-50-109282 proposes to polymerize a vinyl monomer in the presence of a polyorganosiloxane containing a vinyl group or allyl group, thereby to produce a graft copolymer having improved impact strength. (The term "JP-A" as used herein means an "unexamined published Japanese patent application".)
JP-A-52-130885 proposes to use a mercapto group-containing polyorganosiloxane in place of the vinyl or allyl group-containing polyorganosiloxane, thereby to improve the impact strength of vinyl polymers.
Further, JP-A-60-252613, JP-A-61-106614, and JP-A-61-136510 propose to polymerize a vinyl monomer in an emulsion of a polyorganosiloxane containing an acrylic group or methacrylic group, thereby to obtain a graft copolymer having excellent impact strength with a high graft efficiency.
However, the polymerization using either a polyorganosiloxane containing a vinyl or allyl group or a polyorganosiloxane containing a mercapto group has a drawback that these polyorganosiloxanes have poor graft reactivity with a vinyl monomer and, hence, the polymerization product is low in apparent graft ratio, which is calculated from the amount of gel formed and which indicates the proportion of the amount of a vinyl polymer grafted on the polyorganosiloxane to the amount of the polyorganosiloxane.
Therefore, an interfacial adhesive strength between the polyorganosiloxanes and the vinyl polymer is low and delamination is remarkable. The graft copolymer obtained, therefore, has a problem that it cannot have good appearance or sufficient impact strength.
On the other hand, when a polyorganosiloxane containing an acrylic or methacrylic group is used, the graft copolymer having an improved impact strength is obtained, but gloss is low and a sufficient appearance cannot be obtained.